Personal computers typically have an integral loudspeaker for reproduction of sound. Depending on the type of computer (desk-top, portable, personal assistant), the types of sound output required and the sound volume needed will change. Different software will also have different dynamic range requirements. Commonly encountered output sounds include a power-on "boot-beep", alert sounds, voice annotation, and multimedia content. Prior art loudspeaker amplifier designs typically use linear analog amplifier circuitry to drive loudspeakers. Such amplifier designs may include bridge-tied outputs which allow single supply voltage operation.
Various alternative amplifier designs have been developed to achieve the same effect as linear analog amplifiers. For example, loudspeaker amplifiers have been developed which employ a switched mode operation. In switched mode operation, the frequency, duty ratio, or both is controlled to achieve the same average current through the loudspeaker as would be achieved by using a linear analog amplifier. A typical switch mode amplifier is shown in FIG. 1.
Referring to FIG. 1, a typical switch mode amplifier 10 includes a circuit 12 for combining an analog sound signal on a first line 14 with a feedback signal on a second line 16. The adjusted sound signal is transmitted to a comparator 18 over a line 20.
Comparator 18 compares the adjusted sound signal to a reference signal received over a line 22. If the amplitude of the adjusted sound signal exceeds the amplitude of the reference signal, the comparator 18 generates a first signal to close a first switch 24, and a second signal to open a second switch 26. If the amplitude of the adjusted sound signal falls below the amplitude of the reference signal, the comparator 18 generates a second signal to open the first switch 24, and the second signal to close the second switch 26.
One side of a speaker load 28 is connected to switches 24 and 26 through a filter circuit 30. Filter circuit 30 consists of an inductor 32 and a capacitor 34. The other side of the speaker load 28 is connected to ground through a resistor 36 and to circuit 12 by line 16. Sound is generated as current flows through speaker load 28 as switches 24 and 26 open and close in response to the signals from comparator 18.
Amplifier 10 is closed-looped. That is, the sound output of speaker load 28 is adjusted responsive to a feedback signal. Since speaker loads typically do not have a constant impedance over bandwidth, feedback signals are used to adjust the sound signal to better linearize the sound output.
Unfortunately, the manufacture of closed-loop amplifiers designed as shown in FIG. 1 is relatively expensive. For internal computer loudspeaker applications, the expense associated with such closed-loop amplifiers is often unjustified, since such applications often do not require the sound quality of other, high-fidelity applications. Another disadvantage of analog amplifiers is the difficulty in compensating for the tolerance deviations in analog components. In light of the foregoing, it is clearly desirable to provide a simpler, less expensive amplifier. Further, it is clearly desirable to provide an amplifier based on digital technology.
Applying specific enhancement techniques to the general problem of loudspeaker amplifier design is difficult because different applications have different sound requirements. For example, some sounds should be played to alert the user independently of ambient noise. Other sounds could be enhanced to increase intelligibility. Intelligibility may be increased, for example, by increasing the gain for otherwise soft sounds, and decreasing the gain for otherwise loud sounds. Other sounds should not be modified in any fashion, but reproduced exactly. Therefore, it is clearly desirable to provide an amplifier which allows a user to change the response of the amplifier to enhance the "presence" or intelligibility of the sound data, but is equally capable of playing the sound back exactly as originally recorded.